Traditional telecommunication systems that connect two or more sites with physical wire or cable are generally limited to relatively low-speed, low-capacity applications. In order to address these limitations, recently developed systems utilize optical fibers. However, fibers still require a physical cable connection. To remove this limitation, systems utilizing the free space transmission of one or more light beams modulated with data have been developed. Systems using such beams may greatly improve data speed and capacity rates, up to 10 Gigabits per second (Gbps), over traditional wire-based systems and, at the same time, may avoid the traditional communication system infrastructure cost of laying fiber cable to physically connect one site in the system to another site.
Instead of cables, a free space optical data communications system typically includes at least one transmit telescope and at least one receive telescope for sending and receiving information, respectfully, between two or more communications sites. Each of these telescopes contains optics comprising at least a mirror or lens. The transmit telescope uses its optics to transmit the light beam to the receive telescope. The receive telescope uses its optics to focus the incoming light beam onto the focal plane of the telescope. Generally, each telescope is communicatively coupled to a communications network or other source/destination of information. In operation, the transmit telescope receives information from its respective network via cable or wireless transmission, and then transmits a light beam modulated with this information to one or more destination receive telescopes. Each receive telescope then relays data to its intended destination in its respective network via a cable or wireless transmission.
The aforementioned free space communications systems would, therefore, appear to have the benefits of reducing costs associated with installing and maintaining physical hard-wired portions of networks while, at the same time, increasing transmission capacity. However, free space optical communications may be hampered by misalignment of the telescopes. For example, since the transmit and receive telescopes may be located a great distance from each other, initial alignment of the telescopes, to insure that the transmitted light beam is incident upon the focal plane of the receive telescope, may be difficult to achieve. Additionally, even if initially aligned, misalignment of the transmit and receive telescopes may result from any displacement of the light beam during transmission or any movement of either the transmit or receive telescopes or their respective physical mounting platforms. As a result of such misalignment, the transmitted light beam may not be incident upon the focal plane of the receive telescope, or may only be partially incident thereupon, leading to a loss or degradation of communications connectivity.
There is increasing interest in the use of free space optical communication methodology for tactical and other aircraft flying in close proximity or formation. Modulated lasers are a preferred transmission method for free space optical communications. However, very precise aiming is required for the laser beam to impinge upon relatively small optical collectors.
FIG. 1 is a schematic diagram illustrating a prior art free space optical communications system 100 used by aircraft for transmitting data. The system 100 may include a modulated laser transmitter 102 for transmitting a laser beam modulated with data, an optical collector 104 for receiving the laser beam, a photodetector 106 for converting the received laser beam to an electrical signal, an amplifier 108 for increasing the electrical signal from a small signal to a large signal, and a communications receiver 110 for processing the amplified electrical signal to obtain the data. The modulated laser transmitter 102 may be located on a first flying aircraft. The optical collector 104, the photodetector 106, the amplifier 108, and the communications receiver 110 may be located on a second aircraft. In order to maintain high-quality communications connectivity between these two aircraft, the transmitted laser beam need be incident upon the optical collector 104. If the transmitted laser beam is not incident upon the optical collector 104 or is only partially incident upon the optical collector 104, there may be a loss or degradation of the communications connectivity. Suppose the distance L between the two aircraft is 1 mile and the height H of the optical collector 104 is 6 inches, then in order for the transmitted laser beam to be incident upon the optical collector 104, the required aiming accuracy a of the modulated laser transmitter 102 is about ±0.0027 degrees. Such precise aiming may be hard to achieve between two flying aircraft.
Thus, it would be desirable to provide a method and system for improving free space optical communication which may effectively solve the aforementioned misalignment problem.